Metal cylinders, which may be provided with coatings of rubber or other materials, must typically be cooled during extrusion processes, in which the molten product produced by an extruder and formed by a spinneret (a flat, round or profiled spinneret) is cooled by means of direct contact with one or more cylinders, either immediately after the extrusion (typical situation of plants in which the “cast” technology is used) or after a first cooling in air (typical situation of plants in which the “blown” technology is used).
The cooling of these cylinders is generally achieved by the passage of a cold fluid inside the cylinders, which are therefore cooled by conduction, and which, in turn, again exploiting the thermal conduction, provide for the cooling of the molten mass cast onto the cylinders.
The need to heat the metal cylinders, where a well-defined temperature must be guaranteed for the freshly extruded film or films coming from a previously formed reel (in the case of converting plants), is equally frequent and important.
There are numerous cases in which the metal cylinders must be heated, which, in turn, again by means of a thermal exchange by direct contact, provide the plastic film in question with heat until it is brought to the desired temperature. An exemplary case are coupling processes between films having a compatible molecular chemical structure, wherein the surface heating of the areas in contact up to a temperature slightly higher than the Vicat temperature provides for a “welding” between the two films, or all thermoforming processes in continuous wherein the film or sheet must be brought close to the softening point to enable a permanent plastic deformation with the support of light forces.
Another application typical of extrusion lines or off-line processes (i.e. starting from reels of film already produced on any type of filming plant) is the embossing of films, i.e. the impression of a particular geometry or pattern due to pressing on the film by a rubberized cylinder against a second cylinder having a surface characterized by that pattern. Also in this case, specific thermal contents, and consequently specific temperatures, must be reached to ensure that the embossing is impressed on the film as a permanent plastic deformation even with the use of relatively low pressure forces.
An extremely important application that requires a heating process of the plastic film up to very specific temperatures is that which imposes a so-called “orientation” on the film, i.e. a permanent plastic deformation suitable for reducing the starting thickness of the film and consequently increasing the other two dimensions (or only one, in the case of “mono-orientation”) by means of a so-called stretching process of the film. This process, like the previous discussed processes, can be performed either starting from reels of previously produced film with any type of extrusion and filming plant, or “online” in said plants, and basically consists in stretching the plastic film in one or two directions, previously brought to temperatures close to the softening point.
This process is utilized for providing the film with specific physico-mechanical characteristics, which, in the absence of orientation or mono-orientation, would be absent or much lower.
By way of example, two representative cases can be mentioned:
1. The mono-orientation of a mixture substantially and preferably consisting of low-density polyethylenes, or linear low-density polyethylenes (also with a metallocene matrix), or random or block or heterophase polypropylene copolymers, or polypropylene homopolymers or other polyolefins or even other thermoplastic materials in general, combined with inorganic fillers such as calcium carbonate, or talc or glass fiber or in any case any other type of filler in general, in percentages ranging from a few units until over 60%.
Said mono-orientation basically (but not exclusively) has the purpose of creating micropores in the polymer chain, otherwise without any solution of continuity, due to “fractures” caused by said inorganic fillers during the controlled lengthening process; as this mechanical stretching process is carried out with the polymeric material at a temperature close to the softening point, and compatibly with the dimension and particle size of the inorganic fillers, the size of these fractures can be very accurately managed, to the point of being able to not overcome the surface tension of various types of liquids.
In this way, it is therefore possible to provide the film in question with very specific “permeability” characteristics to gases but not to specific liquids, so as to retain the same but not any possible gases.
This, for example, is the typical process used in the production of so-called “breathable” or transpiring films, used more and more frequently in applications for films in the sanitary sector.
2. The mono-orientation of a mixture substantially and preferably consisting of low-density polyethylenes, or linear low-density polyethylenes (also with a metallocene matrix), or random or block or heterophase polypropylene copolymers, or polypropylene homopolymers or other polyolefins or even other thermoplastic materials in general.
In this case, the orientation or mono-orientation is utilized for mechanically characterizing the film in question above all (but not exclusively) from the point of view of tensile strength and breaking strength.
In particular, in the case of film prevalently and preferably consisting of random or block or heterophase polypropylene copolymers, or polypropylene homopolymers, the particular conformation of the polymer chain ensures that the same, if subjected to an orientation action, exponentially increases its mechanical characteristics of tensile strength in the same orientation direction.
Without entering into a physico-chemical explanation of the phenomenon (essentially due to the Van Der Waals bonding forces between the molecules of the polymer), it is sufficient to say that depending on the type of material and type of lengthening ratio effected, the tensile strength characteristics can also increase by several tens of times.
What is indicated above clearly illustrates the importance and diffusion of the necessity of heating films of plastic material up to specific temperatures, preferably and basically by direct contact with rotating cylinders, in turn heated.
The heating of these cylinders is effected in the state of the art by means of hot fluids, whether they be heated or overheated water (therefore also at temperatures higher than 100 degrees, if pressurized), oil, vapour or any other type of fluid, which heats the cylinders directly by internal contact.
This process obviously implies the presence of a heating unit outside the cylinder in contact with the plastic film, wherein the fluid in question is brought to the desired temperature by means of electric resistances, or open flame, or gas, or any other heating system.
The fluid is then transported to the cylinder in contact with the plastic film by means of adequate recirculation systems, such as centrifugal pumps or gear pumps, screw- or piston-compressors, etc., the temperature of the same is then measured by means of suitable reading systems (thermometers, pyrometers), which provide the management system of the same with the reference signal.
This process, common to all applications relating to extrusion or converting plants for plastic films, has various evident and unsolved problems:
1. Poor energy efficiency: in addition to the efficiency of the electric resistances and dispersion of part of the heat supplied in the environment (any insulating system may not be absolute, in addition to preventing or in any case making normal maintenance operations difficult), there are also thermal dispersions in the environment linked to the transportation system of the hot fluid from the heating unit to the cylinder in contact with the plastic film, which is clearly greater, the greater the distance between said heating unit and the cylinder(s).
2. Temperature control: in the systems described above, the temperature of the heating fluid is revealed, and the management of the thermoregulation relates to this signal. What is not controlled, therefore, is the temperature which is most important for the process, i.e. that of the plastic film.
The presence of numerous thermal passages (from the heating unit to the carrier fluid, from the carrier fluid to the cylinder in contact with the plastic film, and finally, as already mentioned, from the cylinder itself to the plastic film) would induce an excessive transition if the whole system were managed by directly detecting the temperature of the plastic film, triggering temperature swings detrimental for the success of the entire process.
3. Complexity: a system such as that described is undoubtedly composed of numerous units and an extremely important plant design, consisting of a heating unit, the transporting system of the carrier fluid, the construction of a cylinder provided with all the features required for a better utilization of the thermal capacity of the carrier fluid (internal double-wall coiling, for best using the caloric content of the carder fluid).
JP 2009 137177A relates to a thermoregulation system of rotating metal cylinders in which infrared heating elements are provided.
WO 2013/160191 A1 relates to cylinders that can be used as operating cylinders of a printing or embossing machine.